1. Field of the Invention
This invention relates generally to membrane mirror assemblies as may be used in adaptive optics systems, and in particular to deforming a membrane mirror used in a wavefront sensor for detecting aberrations in light signals.
2. Background of the Invention
There is an increasing interest in the use of free-space optical communications for various applications. For example, much of the current telecommunications infrastructure is based on the transmission of optical signals via optical fibers. While the use of fiber optics has increased the capacity and efficiency of data transmission, there are many situations where the installation of new fiber is not the best solution. As a result, there is interest in augmenting the telecommunications infrastructure by transmitting optical signals through the free-space of the atmosphere.
Free-space optical communications links can also be used advantageously in applications outside of the telecommunications infrastructure. Compared to other communications technologies, a free-space optical communications link can have advantages of higher mobility and compact size, better directionality (e.g., harder to intercept), faster set up and tear down, and/or suitability for situations where one or both transceivers are moving. Thus, free-space optical communications links can be used in many different scenarios, including in airborne, sea-based, space and terrestrial situations.
However, in many of these potential applications, the free-space optical communications link suffers from optical aberrations. For example, changes in atmospheric conditions can be a significant impediment to the accuracy, reliability, and efficiency of free-space optical communications systems. Wind, heat waves, man-made pollutants, and other effects can create constantly changing aberrations. This, in turn, can degrade the quality of the optical signal that is available at the receiver, resulting in degradation of the overall quality and efficiency of the communications channel.
To address the problem of optical aberrations, adaptive optics systems have been developed to compensate for these aberrations, thus improving the performance of free space optical communications systems. In addition to free-space optical communications, adaptive optics systems can be applied in other areas where optical aberrations are also problematic, such as in telescope imaging systems.
Many adaptive optics systems have a wavefront sensor, which senses the aberrations in the wavefront of received light waves. Existing methods and devices for sensing and measuring the wavefront include several interferometric techniques, the Shack-Hartmann wavefront sensing techniques, and various other systems that involve the projection of patterns of light through an optical system. Once the wavefront senor has measured these aberrations, it can provide a signal to a device for correcting the aberrations, such as a deformable mirror. By adaptively deforming to compensate for the measured aberrations in the light waves, the optical system can correct for these aberrations.
In some wavefront sensors, a modulation device adds a focus (spherical phase factor) term to the incoming light signal at an image plane. To do this, for example, a membrane mirror introduces a dither in the optical path, and the wavefront sensor evaluates the wavefront based on the dithered signal. This can be accomplished using an acoustically driven membrane mirror, using air pressure to deform the membrane mirror and cycle it between convex and concave positions. However, such mirrors are difficult to manufacture in a small form factor and tend to be somewhat unstable over time. Additionally, an acoustic driving mechanism cannot work in environments with little or no air, such as in space and high flying aircraft applications. Acoustic systems also suffer from poor performance due to complicated resonance peaks at various frequencies. Moreover, acoustic systems are highly sensitive to their environment, where changing temperatures and/or pressures during operation further complicates the performance characteristics of the wavefront sensor.
To obtain the spherical deformation for the correct operation of the wavefront sensor, it is desirable to exert a uniform force or pressure on the membrane surface of the mirror. Although acoustic drivers tend to exert this uniform pressure, they suffer from the drawbacks outlined above. Accordingly, it is desirable to produce a sufficient uniform pressure on membrane mirror as desired for a wavefront sensor without suffering from the deficiencies of acoustically driven membrane mirrors.